1. Field of the Invention
The present invention relates to an exchange coupled film which includes an antiferromagnetic layer and a ferromagnetic layer and in which the magnetization direction of the ferromagnetic layer is pinned in a predetermined direction by an exchange coupling magnetic field (Hex) generated at the interface between the antiferromagnetic layer and the ferromagnetic layer, and to a magnetic sensing element including the exchange coupled film. More particularly, the invention relates to an exchange coupled film in which read characteristics, such as the rate of change in resistance and the unidirectional exchange bias magnetic field (Hex*), and reliability can be improved, and to a magnetic sensing element including the exchange coupled film.
2. Description of the Related Art
FIG. 18 is a partial sectional view showing the structure of a conventional magnetic sensing element, viewed from the surface facing a recording medium.
As shown in FIG. 18, an antiferromagnetic layer 30 composed of PtMn or the like is formed on an underlayer 14 composed of Ta or the like.
A pinned magnetic layer 31 is formed on the antiferromagnetic layer 30. The pinned magnetic layer 31 has a laminated ferrimagnetic structure including three sublayers, i.e., a first magnetic sublayer 34, a second magnetic sublayer 36, and a nonmagnetic intermediate sublayer 35. The magnetic sublayers 34 and 36 are composed of, for example, a CoFe alloy, and the nonmagnetic intermediate sublayer 35 is composed of Ru or the like.
A nonmagnetic material layer 32 composed of Cu or the like is formed on the pinned magnetic layer 31, and a free magnetic layer 33 composed of NiFe or the like is formed on the nonmagnetic material layer 32. A protective layer 7 composed of Ta or the like is formed on the free magnetic layer 33.
Hard bias layers 5 are formed at both sides in the track width direction (in the X direction) of the laminate including the underlayer 14 to the protective layer 7, and electrode layers 8 are formed on the hard bias layers 5.
In the magnetic sensing element, when an exchange coupling magnetic field (Hex) is generated between the first magnetic sublayer 34 and the antiferromagnetic layer 30 by annealing treatment in a magnetic field, the first magnetic sublayer 34 in contact with the antiferromagnetic layer 30 is magnetized, for example, in the Y direction, and the second magnetic sublayer 36 in contact with the nonmagnetic material layer 32 is magnetized in a direction opposite to the Y direction by the coupling magnetic field due to the RKKY interaction between the second magnetic sublayer 36 and the first magnetic sublayer 34.
The magnetization direction of the free magnetic layer 33 is aligned in the X direction by a longitudinal magnetic field from the hard bias layers 5.
When the pinned magnetic layer 31 has the laminated ferrimagnetic structure as shown in FIG. 18, the second magnetic sublayer 36 actually contributes to the magnetoresistance effect.
Consequently, when a sensing current from the electrode layer 8 mainly flows through the nonmagnetic material layer 32, if the sensing current shunts to the first magnetic sublayer 34, shunt loss occurs, resulting in a decrease in the rate of change in resistance (ΔR/R).
In order to reduce the shunt loss, an attempt was made in which Cr or the like was incorporated with the first magnetic sublayer 34 composed of a CoFe alloy or the like to increase the resistivity of the first magnetic sublayer 34.
As a result, the resistivity of the first magnetic sublayer 34 was increased, the sensing current from the electrode layer 8 did not easily shunt to the first magnetic sublayer 34, and the rate of change in resistance was improved. However, the unidirectional exchange bias magnetic field (Hex*) was significantly decreased.
Herein, the unidirectional exchange bias magnetic field (Hex*) is the combination of the exchange coupling magnetic field (Hex) primarily generated between the antiferromagnetic layer 30 and the first magnetic sublayer 34 and the coupling magnetic field due to the RKKY interaction generated between the first magnetic sublayer 34 and the second magnetic sublayer 36.
FIG. 19 is a graph which shows the relationship between the thickness and the unidirectional exchange bias magnetic field (Hex*) of the first magnetic sublayer 34 composed of CoFeCr5at% in the magnetic sensing element having the same layered structure as that shown in FIG. 18.
As shown in FIG. 19, when the first magnetic sublayer 34 is composed of a CoFe alloy, a significantly high unidirectional exchange bias magnetic field (Hex*) is exhibited. On the other hand, when the first magnetic sublayer 34 is composed of CoFeCr, the unidirectional exchange magnetic field rapidly decreases.
As shown in FIG. 19, as the thickness of the first magnetic sublayer 34 increases, the unidirectional exchange bias magnetic field (Hex*) increases. However, when the thickness of the first magnetic sublayer 34 is increased, the amount of shunting of the sensing current to the first magnetic sublayer 34 increases. After all, in the conventional magnetic sensing element, it was not possible to improve the unidirectional exchange bias magnetic field (Hex*) and the rate of change in resistance (ΔR/R) simultaneously in the pinned magnetic layer 31.
Additionally, when the unidirectional exchange bias magnetic field (Hex*) is decreased, the pinned magnetic layer 31 cannot be pinned appropriately, the magnetic sensing element is easily affected by electrostatic discharge (ESD), and current-carrying reliability is decreased.
In the magnetic sensing element having the structure shown in FIG. 18, as the track width is decreased, the magnetization direction of the side regions in the track width direction of the pinned magnetic layer 31 is inclined with respect to the height direction (Y direction) under the strong influence of the longitudinal bias magnetic field from the hard bias layers 5. Due to the inclination of magnetization, read output decreases and asymmetry deteriorates. Such a problem is also caused in the magnetic sensing element having the exchange bias system. In the exchange bias system, annealing treatment is performed twice in different magnetic fields. Even if the magnetization of the pinned magnetic layer 31 is pinned in the height direction by first annealing treatment in the magnetic field in the height direction, when second annealing treatment is performed in the track width direction, the magnetization of the pinned magnetic layer 31 is inclined from the height direction.
As the recording densities are improved, the element height of the magnetic sensing element is decreased, and the magnetic sensing element becomes susceptible to electrostatic discharge (ESD), etc.